1. Field of the Invention
The invention relates generally to semiconductor processing. In particular, the invention relates to wafer support fixtures used in batch-mode chemical vapor deposition.
2. Background Art
The fabrication of silicon integrated circuits typically involves one or more steps of chemical vapor deposition (CVD). Many advanced deposition processes use plasma enchanced CVD to activate the chemical reaction resulting in the deposition of the film from a precursor gas. The plasma process allows low temperature deposition. On the other hand, thermal CVD is performed at elevated temperatures to thermally activate the chemical reaction resulting in the deposition of the film from a precursor gas. The temperatures associated with thermal CVD tend to be much higher than those for plasma enhanced CVD, but thermal CVD temperatures fall within a wide range depending upon the material being deposited and the precursor gas.
Thermal CVD is typically utilized for the deposition of silicon nitride and polysilicon. The silicon nitride is used, for example, for etch stop layers and anti-reflective coatings. Silicon nitride has a nominal composition of Si3N4, but some compositional variation is expected, such as SiNx, where x ranges between 1.0 and 1.5. Polysilicon is polycrystalline silicon. It is used for anti-reflective coating and, when doped, for interconnects and electrodes.
Despite the trend to single-wafer processing chambers, batch processing for thermal CVD continues to be widely practiced because of its high throughput and the relatively low cost of equipment. Furthermore, thermal CVD can produce highly uniform films in batch processing. In batch CVD processing, a large number of silicon wafers are loaded onto a support fixture that is placed into a thermal CVD reactor. Typically, the support fixture is a tower in which the multiple wafers are supported horizontally and spaced vertically apart. Some applications continue to use boats as support fixture in which the multiple wafers are supported substantially vertically and spaced horizontally apart.
In the case of the deposition of silicon nitride, the precursor gas is typically composed of silane or a chlorosilane and a nitrogen source such ammonia. At elevated temperatures, typically in the range of 600 to 800° C. but sometimes extending down to 400° C. or even lower, the precursors react near the surface of the wafer to deposit silicon nitride on the wafer surface. In the case of chlorosilane and ammonia precursors, the reaction products are Si3N4 and NH4Cl. The former deposits on the wafer while the latter is volatile and is evacuated from the furnace. However, thermal CVD tends to coat all surfaces exposed in the furnace. In particular, the support tower is typically coated with as much silicon nitride as is the wafer.
Quartz has in the past been the most prevalently used material for support towers used in a thermal CVD process. Quartz has a chemical composition of amorphous silicon dioxide, which is compatible with most silicon processing. At the relatively low temperatures usually experienced in CVD, whether thermal or plasma enhanced, quartz remains in a glassy state with a very smooth surface so that it is a very clean material. However, as the feature sizes on integrated circuits has decreased to 0.18 μm and even smaller, quartz support towers have nonetheless experienced substantial problems with producing particles. These particulates fall on the wafer and can significantly reduce the yield of operable integrated circuit dies obtained from the wafer.
Often integrated circuit fabrication is monitored by measuring the number of particles added to a wafer by any step of the fabrication process. It has been found in thermal CVD of silicon nitride that the number of particles generally increases with the number of runs or batches that the quartz tower has processed. As illustrated in FIG. 1, a new tower produces relatively few particles. Thereafter, the number of particles increases with the number of runs, but up to about forty runs the number is acceptable, though still somewhat high. However, after some number of runs, the number of particles greatly increases to a totally unacceptable level. It is believed that the origin of the problem is that the silicon nitride is also depositing on the quartz tower. For 40 runs of depositing 0.15 μm of silicon nitride, a typical nitride layer thickness in an integrated circuit, the nitride build up on the tower may be 6 μm. Silicon nitride has a coefficient of thermal expansion that is significantly different than that of quartz, about 3×10−6 versus 0.5×10−6/° C., and the nitride does not bond well with the glassy quartz surface. Differential thermal expansion between the two materials as the tower is cycled between room temperature and the relatively modest thermal CVD temperatures causes the thickly deposited nitride to peel from the quartz and to produce nitride particles, some of which settle on the wafers.
For these reasons, it is typical practice in a production environment to use a tower only for a number of runs somewhat below the experimentally determined point at which the particle count rapidly increases, for example, thirty runs for the data displayed in FIG. 1. It is common practice to then clean the quartz tower in bath of hydrofluoric acid and nitric acid to remove the silicon nitride and to return the cleaned tower to service for another cycle of runs. However, the baseline particle count for a cleaned tower is somewhat higher than that for a new tower, and the number of runs before onset of unacceptable particle count is reduced by about 25%. As a result, quartz towers are typically discarded after only two or three cycles. Although quartz towers are relatively inexpensive, such short life greatly increases the cost of ownership (COO) when measured per wafer. Also, the necessity of changing out towers and cleaning towers complicates the work flow and reduces productivity.
Furthermore, the baseline particle counts for quartz towers are still high, and the onset of greatly increased counts is somewhat variable. Both factors reduce the yield of operable dice obtained from the wafers.
Some of these reasons have prompted the use of silicon carbide towers. Bulk silicon carbide is typically formed by a sintering process, which produces a material containing a high fraction of impurities. For this reason, the sintered material is usually covered with a layer of CVD silicon carbide. As long as the CVD layer is not punctured, contamination is not a problem. The peeling problem is not totally eliminated, but if, after a nitride buildup of about 20 μm on the silicon carbide tower, it is cleaned in aqua regia (HF/HNO3) for up to a week, the tower can be used almost indefinitely. However, the CVD silicon carbide film is fragile, and a single pin hole through the film ruins the coating protect so the tower must be scrapped. Entire towers of CVD silicon carbide can be made, but they are very expensive.
Accordingly, it is greatly desired to provide a support tower that is not subject to such particle problems and can be used for many more runs without cleaning or replacement.